This invention stems from Michelson's experiment in which he measured the rotation of the earth by the Sagnac effect. Michelson used an evacuated tube buried underground and laid out in a 6200 foot rectangle, and he was able to detect a shift of 0.230 of a fringe of light having a wave length of 5700 A (Ditchburn LIGHT , p.339). This was within 1% of the calculated value and demonstrated that an optical gyroscope of high accuracy is feasible. Michelson's device, of course, was too large to be useable as a practical matter, but his success has stimulated efforts ever since then to reduce it in size in order to make it practical. So far it has not been achieved. It was an object of the specific embodiments described in my prior patent, but as I pointed out therein, noise was a major problem. This invention has essentially the same objectives.
The state of the art in this field is discussed by Dana Z. Anderson in the April 1986 issue of Scientific American. In particular he describes using long lengths of optical fibers to maximize the Sagnac effect. Such usage has long been an obvious choice ever since good optical fibers came on the market in the late 1970's, but a practical device using them has yet to be developed as Mr. Anderson notes in his article.
One problem is that laser light has a tendency to lose its phase coherence after it has travelled more than a few dozen meters. This makes it difficult to obtain accurate measurements using long lengths of optical fibers by detecting the nodes or loops of a laser-formed standing wave. On the other hand, optical glass has another property which can be used to advantage, i.e. the Fizeau effect and the Fresnel convection coefficient whereby the velocity of light in a moving medium is changed due to motion of the medium. It is a way by which the delay required in the mode of operation of my said prior patent may be introduced using the rotation of the platform as the source of motion.
It is an object of this invention to make an improved optical gyroscope which overcomes the disadvantages of the prior art and amplifies both the Sagnac and Fizeau effects to the point where a practical device can be made. A further object is to provide an optical gyroscope in which the Fizeau effect is maximized within the limits of preserving the accuracy of measurement by the use of phase-coherent light.